1. Field of the Invention
The present invention generally relates to an optical recording/reproducing apparatus, and more particularly to an equalizer and an equalizing method in which the inter-symbol interference of reproduced signal passed through channel is removed in reproducing apparatus that reproduces data recorded on recording medium.
2. Description of the Related Art
Recently, with the advance of multimedia information, as demands on the storage and the transmission of digital data having a greater capacity are on the increase, the studies on optical recording media such as DVD have actively progressed. At the present, DVD market has been grown on the basis of computer industry desiring to adopt DVD-ROM and home appliance industry wishing to adopt DVD-Video. Besides, write-once type DVDR, rewritable type DVD-RW, and DVD-RAM are on the market, and hence the application fields of the optical recording medium have been expanded. In addition, in the future, studies with respect to the HDTV have actively progressed. HD-DVD has a size equal to that of CD, DVD, but has a storage capacity above 15 GB. By the use of a blue laser and the use of less track pitch and minimal pitch length, a higher density channel is possible. However, since the inter-symbol interference increases with the density of the channel information being increased, the quality of the reproduced signal is degraded and hence there exist many difficulties in correctly reproducing the recorded data.
In order to solve the foregoing problems described above, conventionally, the reproduced data having the inter-symbol interference (ISI) generated through the channel is outputted as a correctly reproduced data through the equalizing process of decision feedback equalizer (DFE) described below.
FIG. 1 is a schematic block diagram showing an equalizer having a general DFE structure for removing the inter-symbol interference (ISI) generated in the reproduced signal of the conventional optical recording/reproducing apparatus.
The equalizer 10 comprises FeedForward (FF) unit 11, FeedBack (FB) unit 12, an adder 13, a slicer 14, and an error calculating unit 15.
FF unit 11 has a feedforward filter, and removes pre-cursor from the inter-symbol interference of the reproduced signal.
FB unit 12 has a feedback filter, and predicts post-cursor of the inter-symbol interference of the reproduced signal.
The adder 13 outputs the signal having the inter-symbol interference removed by adding the reproduced signal having the pre-cursor removed by the FF unit 11 and the reproduced signal having the post-cursor predicted by the FB unit 12.
The slicer 14 decides the level of the signal added at the adder 13 to be a predetermined level, and outputs the level-decided signal. The level-decided signal output from the slicer 14 is fed back to FB unit 12.
The error calculating unit 15 calculates equalizing error value by using the signal from the adder 13 and the signal from the slicer 14. The equalizing error value calculated is then inputted to FF unit 11 and FB unit 12, where the tap coefficients of the respective filter tap are updated in accordance with the inputted equalizing error value.
The conventional equalizer having DFE structure described above has an advantage in that a fast equalizing speed be can obtained with a relatively less number of taps, but there is a disadvantage in that the reliability of the signal inputted to the feedback filter, i.e., the signal level decided at the slicer 14 degrades, and accordingly, the equalizing performance degrades.
In order to solve the defects of the equalizer of such DFE structure, the equalizer 20 shown in FIG. 2 is used wherein the signal with the level decided by FDTS/DF (Fixed Delay Tree Search with Decision Feedback) method and FDTrS/DF (Fixed Delay Trellis Search with Decision Feedback) method is inputted to the feedback filter.
The equalizer 20 has the equalizer (DFE) 10 shown in FIG. 1 added with a signal detection unit 26 of FDTS/DF method or FDTrS/DF method.
FF unit 21 removes pre-cursor from the reproduced signal by the feedforward filter.
FB unit 22 predicts and outputs post-cursor of the inter-symbol interference of the input signal, i.e., the reproduced signal by the feedback filter.
A first adder 23 removes the inter-symbol interference from the input signal by adding the signal having the pre-cursor removed by the FF unit 21 and the post-cursor predicted by the FB unit 22.
The slicer 24 decides the level of the signal outputted from the first adder 23 to be a predetermined level and outputs it. The error calculating unit 25 calculates equalizing error value based on the signal from the first adder 23 and the signal from the slicer 24. The equalizing error value calculated is inputted to FF unit 21 and FB unit 22, where the tap coefficients of the respective filter are updated based on the inputted equalizing error value.
The signal detection unit 26 outputs τ signals of decided levels using any one of FDTS/DF method and FDTrS/DF method having a fixed decision delay (τ), and feedbacks the signal with the decided level to FB unit 22. FB unit 22 predicts the remaining post-cursors except for τ post-cursors and outputs them to a second adder 27.
The second adder 27 adds the signal having the pre-cursor removed by the FF unit 21 to the post-cursor predicted by FB unit 22 based on the feedback signal of the signal detection unit 26 and outputs the added signal. The signal output from the second adder 27 is inputted to the signal detection unit 26, and outputted as a predetermined level signal.
In accordance with the conventional equalizer having the configuration above, it is possible to enhance the equalizing performance by inputting the signal in the level decided by FDTS/DF method or FDTrS/DF method of the signal detection unit 26 to the feedback filter. However, due to additional computational amount caused by the operational characteristic of the signal detection unit 26 of FDTS/DF method or FDTrS/DF method, there is generated a substantial amount of time delay in deciding the signal level. Accordingly, there exists a problem in that the overall operating speed of the equalizer degrades.